White light spectrum is a successive spectrum, and the white light viewable by human eye is formed by mixing lights with at least two or more colors (wavelengths), for example, the white light can be formed by mixing three primary colors (red light+green light+blue light), or can be formed by mixing complementary colors, such as (blue light+yellow light) or (cyan light+red light). According to the principle of forming the white light, recently, the white light emitting diodes (LEDs) can be approximately classified into two types. The first type is a white LED of three primary color type, which is composed of three semiconductor chips respectively emitting red light, green light, and blue light, and is also referred to as a multi-chip white LED or a triple wavelength white LED. The other type is a white LED of complementary color type, in which a single LED chip emitting a light ray with a single color is used, and fluorescent powder capable of being excited by the color light ray to emit a light ray with a color complementary to that of the color light ray, so it is also referred to as a single-chip white LED. However, when using the multi-chip white LED, the difference of the semiconductor materials results in the difficulty on the design of drive circuit.
Referring to FIG. 1, a schematic diagram of a conventional single-chip white LED is shown. In the single-chip white LED, fluorescent powder and gel are formulated according to a specific ratio to form a light conversion filling 10, and the light conversion filling 10 directly surrounds an LED chip 13 and fills up a cup 14. Therefore, when the LED chip 13 emits light, generated light pass through the light conversion filling 10, and blue light emitted by the LED chip 13 and yellow light excited from the light conversion filling 10 by the blue light are mixed to obtain the white light. For the light emitted in different angles from the LED chip 13, the paths to the light conversion filling 10 vary, and the distances for the light to pass through the light conversion filling 10 are different. The light ray 111 with a larger incident angle passes through the light conversion filling 10 and is redirected by the reflective wall 15. The path distance of passing through the light conversion filling 10 is longer; therefore, the energy is mostly converted to the yellow light excited from the light conversion filling 10 and forms the white light similar to the yellow light on the periphery of the cup 14. Accordingly, the single-chip white LED shows an annular yellow light region on the periphery of the emitted white light, that is, halo phenomenon. The light ray 112 with a smaller incident angle emits upwardly without being redirected, so that the path of passing through the light conversion filling 10 is shorter, and the energy is mostly at the blue light emitted by the LED chip 13. Consequently, when the single-chip white LED is measured, it is found that color temperatures are different at different positions, and the difference of the color temperatures can be up to 700 K at most, thus resulting in a problem of non-uniform color temperature of the emitted light ray.
Referring to FIG. 2, a schematic diagram of a conventional multi-chip white LED is shown. In the multi-chip white LED, fluorescent powder and gel are formulated according to a specific ratio to form a light conversion filling 20, and the light conversion filling 20 directly surrounds LED chips 18a, 18b, and 18c, and fills up a cup 19. In addition to the halo phenomenon mentioned in the single-chip white LED, the phenomenon of non-uniform color temperature is obvious to be detected by viewing from the top of the cup 19, since mostly the light emitted from the surface of the chip in the multi-chip white LED passes through the light conversion filling 20 upwardly, whereas the light emitted by two chips pass through the light conversion filling 20 in-between chips at the same time, so that the color temperature of the light emitted between the chips is lower than that of the light emitted from the surface of chip.
Theoretically, same color temperature could be achieved by having the emitting blue light pass through the equal distance of the fluorescent material. Accordingly, solutions as to forming the uniform thickness of fluorescent material are fervently discussed and proposed. In the U.S. Pat. No. 5,959,316, entitled “Multiple en-capsulations of phosphor-LED devices”, a transparent hemispherical spacer of resin is formed and fixed first for fluorescent material to flow to conform to shape. The layer of fluorescent material is claimed to be uniform thickness; however, arrangement as mentioned will make the manufacturing process difficult to implement. In the U.S. Pat. No. 7,129,638, entitled “Light emitting devices with a phosphor coating having evenly dispersed phosphor particles and constant thickness”, a fixed fluorescence conversion layer is formed by uniformly sub-siding the fluorescent powder with gel on the LED chip before the gel is cured. The material selection and complex manufacturing process will limit the application. In the U.S. Pat. No. 6,650,044, entitled “Stenciling phosphor layers on light emitting diodes”, the problem of unequal color temperature is effectively resolved by disposing the phosphor layer of uniform thickness around the chip surface through conformal packaging. Due to the manufacturing process, the eligible method of packaging will be limited to flip chip, waiving the use of wire bond, so that the breaking of wire bond could be saved. In the Japan patent 2004-179644 “PHOSPHOR LAMINATION AND LIGHT SOURCE USING THE SAME”, the problem of unequal color temperature is also effectively resolved by adhering the fluorescence conversion layer with uniform thickness to the chip surface through screen printing. This conformal packaging again suits merely to the flip chip owing to the manufacturing process. In the patent US2006/0003477 (WO2004040661) entitled “METHOD FOR PRODUCING A LIGHT SOURCE PROVIDED WITH ELECTROLUMINESCENT DIODES AND COMPRISING A LUMINESCENCE CONVERSION ELEMENT”, the desired effect of same color temperature, though possible, is achieved painstakingly by controlling the machines and manufacturing process precisely. Furthermore, the manufacturing process has to be executed directly on the LED chip.